Roadmap for the development of control strategies for liver fluke

What are we trying to achieve and why? What is the problem we are trying to solve?

Can we intervene against the “carrier state” to improve fluke control?
Is the carrier state an appropriate concept for fluke infections, in which low level infections produce eggs, which are then amplified considerably in the intermediate host, and therefore contribute actively to on-farm epidemiology and infection pressure. This is very different to the idea of carrier states in other animal diseases. Low level infections can also have production impacts. These are more appropriately defined on a continuum of fluke burden, than being falsely categorised as latent or hidden.

What are the scientific and technological challenges (knowledge gaps needing to be addressed)?

The sub-clinical carrier state is dominant in fluke infections, since infection is widespread within infected herds but most animals are sub-clinically infected. Long duration of patent infection means that apparently healthy animals contribute to pasture contamination. Identification and elimination of these infections is unlikely to be any more productive than existing whole-herd chemical intervention strategies. Rather, it might be useful to determine whether animals differ in the threshold level of infection that incurs production losses, and whether this can then be manipulated to, e.g. by selective breeding.

What approaches could/should be taken to address the research question?

Studies in experimentally and naturally infected animals that more precisely elucidate the relationship between fluke burden and production impacts, including intervention studies based on different indicators and treatment thresholds. Hence determine whether tolerance of infection below a given threshold is a viable strategy, and whether variation in this level between individuals and breeds is heritable and provides a route for selecting fluke-tolerant production animals.

What else needs to be done before we can solve this need?

Allowing persistence of the carrier state in some animals carries a danger of enabling persistent shedding of eggs into the environment and increased infection pressure across the herd and farm. This would have to be managed effectively for this strategy to be viable. There is significant danger that tolerating low levels of infection could rapidly lead to increases in parasite abundance and production impacts due to amplification in snail hosts.

Existing knowledge including successes and failures

To date it has largely been assumed that elimination of fluke, insofar as it is possible, is the aim of control. This is changing as existing control methods are recognised to be only partly effective, and management of drug resistance raises the question of whether generation of refugia is appropriate for fluke. This could lead to more critical appraisal of the carrier state (=sub-clinically infected individual) and management of fluke that recognises the inevitability of this state on infected farms.